Benchmark 4D
The Physical Setting: Structure of Matter
Grades 9-12, page 80

The nucleus of radioactive isotopes is unstable and spontaneously decays, emitting particles and/or wavelike radiation. It cannot be predicted exactly when, if ever, an unstable nucleus will decay, but a large group of identical nuclei decay at a predictable rate. This predictability of decay rate allows radioactivity to be used for estimating the age of materials that contain radioactive substances.
 

NSES Content Standard B  Physical Science: Structure of atoms  Grades 9-12, page 178  Radioactive isotopes are unstable and undergo spontaneous nuclear reactions, emitting particles and/or wavelike radiation. The decay of any one nucleus cannot be predicted, but a large group of identical nuclei decay at a predictable rate. This predictability can be used to estimate the age of materials that contain radioactive isotopes.  NSES Content Standard D  Earth and Space Science: Origin and evolution of the earth system  Grades 9-12, page 189  Geologic time can be estimated by observing rock sequences and using fossils to correlate the sequences at various locations. Curent methods include using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.